1. Field of the Invention
The invention relates to the process of manufacturing an integrated circuit. More specifically, the invention relates to a method and an apparatus for exposing a wafer using multiple masks during an integrated circuit manufacturing process.
2. Related Art
Integrated circuits can be produced through an optical lithography process that involves creating a mask with a pattern specifying where the various features of the integrated circuit are to be placed and then passing radiation through the mask to expose the pattern on a semiconductor wafer. This pattern defines where the surface of the semiconductor wafer is to be etched or where new material is to be added to create the integrated circuit.
FIG. 1 illustrates the process of exposing a target wafer through a mask. As shown in FIG. 1, radiation emanates from source 102, which can be any suitable source of radiation such as a laser. This radiation passes through illumination optics 104, and then through mask 106, to imaging optics 108. Mask 106 includes the circuit patterns to be projected onto target 110. Imaging optics 108 directs the radiation, including the circuit patterns onto target 110. Target 110 is typically a semiconductor wafer treated to be sensitive to the radiation being used. The above-described process transfers the circuitry delineated on mask 106 to target 110.
As features on integrated circuits become progressively smaller, designers have resorted to splitting the mask into multiple parts so that features can be placed on the separate masks to improve resolution and reduce interference between various features of the circuit and simplify mask manufacturing.
Using multiple masks requires that each of the masks is used to expose the semiconductor wafer. There are several ways that this might be accomplished. One possibility is to use two steppers and to expose the semiconductor wafer through a first mask in the first stepper, and then to expose the semiconductor wafer through a second mask in the second stepper. Since steppers are very expensivexe2x80x94several million dollars eachxe2x80x94using two steppers is a very expensive option.
Another possibility is to use two reticle holders within a single stepper, where each reticle holder can be moved into a position where the beam of radiation is passed through the first mask for a first exposure and then passed through the second mask for the second exposure. While this eliminates the expense of having two steppers, changing reticle holders slows the manufacturing process and reduces throughput. Note, the terms xe2x80x9cmaskxe2x80x9d and xe2x80x9creticlexe2x80x9d are used interchangeably throughout this application.
What is needed is a method and an apparatus that allow multiple masks to be used for exposing a semiconductor wafer without the problems discussed above.
One embodiment of the invention provides a system that facilitates exposing a wafer through at least two masks during an integrated circuit manufacturing process. The system includes a radiation source and two or more illuminators. Each of these illuminators receives radiation from the radiation source, and uses the radiation to illuminate a reticle holder. The radiation that passes through each reticle holder is then combined in an optical combiner, before passing through an imaging optics, which projects the combined radiation onto a semiconductor wafer.
In one embodiment of the invention, the radiation source includes two or more sources so that each illuminator receives radiation from a different source.
In one embodiment of the invention, the radiation source includes a beam splitter that splits the radiation from the radiation source into a separate beam for each illuminator.
In one embodiment of the invention, although there are more complex illumination pathways, only a single imaging optics is used for an overall saving. The invention provides improved throughput of wafers in fabrication and better processing control since the time between both exposures and processing is shorter.
In one embodiment of the invention, the system includes a phase shifting device interposed in the path of the radiation in one or more of the illuminators so that the phase of the radiation in one illuminator can be adjusted relative to the phase of the radiation in the other illuminators. Adjusting the relative phase of the radiators may reduce mask fabrication costs as described below.
In one embodiment of the invention, adjusting the phase of the radiation in one illuminator relative to the phase of the radiation in the other illuminators allows splitting a dark field mask that includes 0-degree and 180-degree phase shifters into two dark field binary masks.
In one embodiment of the invention, the system includes adaptive optics at the optical combiner to locally adjust for differences in phase between the illuminators and the objects (masks) at each point within the field.
In one embodiment of the invention, the system includes an optical path-lengthening device interposed in a path of radiation within at least one illuminator. This optical path-lengthening device causes temporal incoherence between the associated illuminator and the other illuminators. Temporal incoherence is useful in cases where only a double exposure is wanted without any phase shift effects.